FJR Center of gravity

[Pete]

Hi,

I have mostly been a lurker on the forum but now I could use some help. I hope you don't mind my jumping in. I've been asked to fabricate a fuel cell for an 2009 FJR and I have a tech question. I've searched and searched but I can't seem to find an answer.

At what height is the center of gravity on the FJR?

I have the 2011 FJR1300A specs from the factory and I am assuming the COG is below the seat height of ~32". I need to know how much the COG will raise by adding 30lbs of fuel in the pillion area. All I have been able to find is the statement "the FJR has a low center of gravity". The only way I know how to find this info is to hoist the bike and find a balance point but the owner might not care much for that.

FYI I am looking to replicate the Tanji Flatbed with a simpler fabrication process and if this is successful you may be hearing more about it.

Thanks for the ear and any help you can provide.

Best regards,

--

Mike Langford [aka IB Pete]

Chula Vista, CA

IBA#341

https://employees.org/~mlangfor/

The mysteries of life?

"it's faster horses, younger women, older whiskey, and more money"

Tom T. Hall

 

[Ignacio]

I don't ever recall anybody actually knowing exactly where the COG is on any Gen bike.

I just know having 41.568 pounds of fuel myself in my pillion area (give or take a bit if you're a Tom Austin) plus aluminum container...doesn't seem to make much difference. I notice more from having 25 pounds on the tail.

 

[TomInPA]

Interesting project, and I wish we were seeing it done for a Gen I bike as well. There are a number of bikes with auxiliary fuel tanks that seem to be capable of insane speeds and good performance in twisties (judging from my efforts to keep up with the pilots). Filling the void behind the driver with a fuel tank also seems to help stability by reducing aerodynamic effects between a driver and the top-case. And if that wasn't enough, the fuel platform can make for a handy conversion to an electric vehicle.

 

[NEPA Jim]

Hi,

I have mostly been a lurker on the forum but now I could use some help. I hope you don't mind my jumping in. I've been asked to fabricate a fuel cell for an 2009 FJR and I have a tech question. I've searched and searched but I can't seem to find an answer.

At what height is the center of gravity on the FJR?

I have the 2011 FJR1300A specs from the factory and I am assuming the COG is below the seat height of ~32". I need to know how much the COG will raise by adding 30lbs of fuel in the pillion area. All I have been able to find is the statement "the FJR has a low center of gravity". The only way I know how to find this info is to hoist the bike and find a balance point but the owner might not care much for that.

FYI I am looking to replicate the Tanji Flatbed with a simpler fabrication process and if this is successful you may be hearing more about it.

Thanks for the ear and any help you can provide.

Best regards,

--

Mike Langford [aka IB Pete]

Chula Vista, CA

IBA#341

https://employees.org/~mlangfor/

The mysteries of life?

"it's faster horses, younger women, older whiskey, and more money"

Tom T. Hall

Well, my guess it that the CG is at some line below the top of the engine given the heavy shaft drive, center lines of the symmetrical wheels etc..... A real quick reality check is that the bike weighs in at 650 lbs or so wet and 30 pounds is less than 5% of the total mass and will only move the cg a slight amount. I would think the sloshing fluid would be more noticeable if not baffled.

 

[SLK50]

Approximate COG, rule-of-thumb if you prefer, is often assumed to be near

or at the level of the crankshaft.

Heavy items like drive shaft, axles, wheels, sump, etc. below the crankshaft

are offset by equally heavy items like fuel tank, handlebars, instruments,

valve gear, etc. above.

 

[Greb]

Interesting project, and I wish we were seeing it done for a Gen I bike as well. There are a number of bikes with auxiliary fuel tanks that seem to be capable of insane speeds and good performance in twisties (judging from my efforts to keep up with the pilots). Filling the void behind the driver with a fuel tank also seems to help stability by reducing aerodynamic effects between a driver and the top-case. And if that wasn't enough, the fuel platform can make for a handy conversion to an electric vehicle.

Damn, look at them fuel prices! No wonder you were trying to convert it to electric!

 

[weekend rider]

Pete, you may want to try and use an equal lateral triangle set up for finding the center of gravity, doing both sides or a complicated math formula

 

[Donal]

Interesting project, and I wish we were seeing it done for a Gen I bike as well. There are a number of bikes with auxiliary fuel tanks that seem to be capable of insane speeds and good performance in twisties (judging from my efforts to keep up with the pilots). Filling the void behind the driver with a fuel tank also seems to help stability by reducing aerodynamic effects between a driver and the top-case. And if that wasn't enough, the fuel platform can make for a handy conversion to an electric vehicle.

Damn, look at them fuel prices! No wonder you were trying to convert it to electric!

If you think that's expensive you ought to try riding over here. It's £1.35 a litre (that's £5.10 a US gallon, or $8.17 per US gallon).

Don

 

[Fred W]

I think the way to look at this is that the Center of Gravity on the unladen bike will be roughly at the visual center of its mass when viewed from the side.

Now when you put a rider and passenger on top, the center of mass (and combined COG) raises significantly. Most aux fuel cells are used only for solo riding. There is no way that an aux fuel cell even when full is going to effect the COG anything near what a passenger does.

On the days when you want to be a bit sportier, just don't fill the aux cell.

 

[Pete]

Hey all,

Thanks so much for all the feedback, this is very helpful. What I am trying to do is separate the fuel cell from the mounting. The Tanji cell (and others I have seen) are a fabrication nightmare and, as has been mentioned, model year specific. I am hoping to be able to build something where the mount is model specific and the cell is generic. As such, it may need to be higher and wider than the Tanji cell.

I was hoping to garner some facts about COG such that I could rationally say how much the COG changes with any design I come up with. I agree completely with the statement that 30lbs of fuel is not even close to a passenger. My son (phd in engineering) guestimated that raising 40lbs an inch might change COG by >.1 inch. I'll have to do some more math to get a better idea.

FWIW here is what I have been doing with my K1200LT where I have separated the platform from the cell.

two different flavors:

rider and pillion -- https://ibpete.blogspot.com/2011/02/latest-fuel-cell.html

single rider -- https://picasaweb.google.com/IBPete/K1200LTSingleRiderFlatTopFuelCell#

Again, thanks for all this great feedback.